Optical disks known as DVD (Digital Versatile Disks) are available on the market as high-density high-capacity optical information recording media. Such optical disks recently have gained rapidly in popularity as recording media that can record images, music and computer data. In recent years, advances have been made in research in next-generation optical disks that have an even higher recording density. Next-generation optical disks are awaited as recording media that are apt to replace the currently prevalent video tapes of VTR (video tape recorders), and their development is advancing at a tremendous pace.
Optical heads that record or reproduce information onto or from an optical disk are provided with a light source, an objective lens that focuses a beam that is emitted from the light source onto an optical disk, and a detector that detects the beam that is reflected from the optical disk. Semiconductor lasers used for the light source emit a beam from the end face of a thin active layer, so that the shape of the beam is elliptical, and the ratio between the short axis and the long axis of the beam is about 1:3. When recording information onto the optical disk, it is desirable to improve the light utilization efficiency by shaping the elliptical beam into a circular beam.
The following is a description of first to fourth conventional examples of beam shaping.
FIG. 16 shows a first conventional example (see, for example, Laid-Open Utility Model Application S63-118714 (FIGS. 1 and 4)) of adjusting the shape of a beam into a circular shape with a lens, and is a diagrammatic view of an optical head 309 using a beam shaping lens 302. An elliptical divergent beam emitted from the light source 301 is shaped into a circular divergent beam by a later-described beam-shaping lens 302, passes through a beam-splitting prism 303, is collimated into a parallel beam by a collimating lens 304, reflected by a mirror 305, condensed by an objective lens 306, and irradiated onto an optical disk 310. The beam reflected by the optical disk 310 travels back along the reverse path, is reflected by the beam-splitting prism 303, and is detected by a detector 308.
Both faces of the beam-shaping lens 302 are cylindrical surfaces, and the beam is refracted and enlarged along its short axis direction by the cylindrical surfaces, whereas the spread angle is not changed along its long axis direction, so that the elliptical beam is shaped into a circular beam.
FIG. 17 shows a second conventional example (see, for example, Laid-Open Utility Model Application S63-118714 (FIGS. 1 and 4)) using a cylindrical lens 302a and a cylindrical lens 302b that are spatially separated from one another. Also with this configuration, an elliptical beam can be shaped into a circular beam, like in the first conventional example.
FIG. 18 shows a third conventional example (see, for example, Laid-Open Patent Application 2002-208159A (FIG. 1)) of adjusting the shape of the beam into a circular shape with a lens. A first surface 402i and a second surface 402o of a beam-shaping lens 402 are cylindrical surfaces, and the beam is refracted and enlarged along its short axis direction by the cylindrical surfaces, whereas it passes through without changing its spread angle along its long axis direction, thus shaping the beam. The first surface 402i is an aplanatic surface, so that no aberration occurs. The distance on the optical axis from the emission point of the light source 401 to the first surface 402i is the same as the thickness of the beam-shaping lens 402 on the optical axis, so that the beam of the short-axis direction is perpendicularly incident on the second surface 402o, and no aberration occurs. The cross-section of the second surface 402o through the plane that is perpendicular to the center axis of the cylindrical surface (the plane parallel to the paper plane is a non-circular arc and in the following, such a cylindrical surface is referred to as a “aspherical cylindrical surface”). With the second surface 402o as an aspherical cylindrical surface, an axially rotation-symmetric spherical aberration is attained by causing aberrations in the short axis direction that are of the same extent as in the long axis direction. The spherical aberration caused by this beam-shaping lens 402 is eliminated by a collimating lens 404.
FIG. 19 shows a fourth conventional example of shaping a beam into circular shape with a prism, and is a diagrammatic view of an optical head 509 using a beam-shaping prism 502. The divergent beam emitted from the light source 501 is collimated into a parallel beam by a collimating lens 504, and an elliptical beam is shaped into a circular beam by refracting the beam along the beam's short axis direction with a beam-shaping prism 502. The circular beam passes through a beam-splitting prism 503, is reflected by a mirror 505, is condensed by an objective lens 506, and is irradiated onto an optical disk 510. The beam that is reflected by the optical disk 510 travels back the reverse path, is reflected by the beam-splitting prism 503, passes through a detection lens 507, and is detected by a detector 508.
However, with the beam-shaping lens 302 of the first conventional example shown in FIG. 16, the beam-shaping magnifying power is limited approximately to a factor of 1.2. The cross-section of the cylindrical surface of the beam-shaping lens 302 through the plane that is perpendicular to the center axis of the cylindrical surface (the plane parallel to the paper plane in FIG. 16) is a simple substantially circular arc (in the following, such a cylindrical surface is referred to as a “spherical cylindrical surface”). When the beam-shaping magnifying power to attain a substantially circular beam is set to at least a factor of 2 or greater, then higher-order aberrations of at least 0.06λ (where λ is the wavelength) occur due to the spherical cylindrical surface, making it impractical.
Moreover, also with the cylindrical lens 302a and the cylindrical lens 302b of the second conventional example shown in FIG. 17, with the beam-shaping magnifying power set to a factor of 2 or greater, the same higher-order aberrations will occur. Moreover, the cylindrical lens 302a and the cylindrical lens 302b are spatially separated, so that there is the problem that, due to temperature changes, their spacing may change and the beam-shaping magnifying power may change, and the aberrations become worse.